The present invention relates to a temperature history indicator formed as a chip, label, tag and the like and its manufacturing method. More particularly, it relates to temperature history indicators bonded to or accompanied with goods or their packings whose preservation or storage temperatures must be controlled so as to display whether or not the goods have been preserved under their corresponding preservation or storage temperatures, and it also relates to a manufacturing method of the same.
Transportation and preservation of frozen or refrigerated foods, natural flowers, bloods, medicines and the like must be carried out under strictly-controlled temperatures which are not necessarily below the freezing point. Even if the temperatures under which these matters are now preserved are correct, it is left unclear whether or not they have been preserved under their correct temperatures. It has been therefore needed to develop a visual indicator for displaying as easily as possible whether or not each of the matters has been preserved under a predetermined temperature.
As temperature history indicators to meet the need, there can be cited a combination of melting phenomenon of dyed solid resinate and capillarity of absorbent paper, which was proposed by U.S. Pat. No. 3,954,011, and a measure of using that a lipase enzyme is activated in degrees varying with temperature and that its degree of activation can be indicated by different colors with the aid of a Ph indicator, as proposed by Japanese Patent Publication Nos. 57-4319 and 57-28559.
However, these indicators are expensive and cannot be easily used as the temperature history indicator which is applied to each of matters circulated in mass. Inventors of the present invention tried variously to solve this drawback and found an extremely characteristic and effective label (see Japanese Patent Preliminary Publication No. 60-053984) wherein the color developing system of using a methine dye-precursor and an oxidizing material was applied to the temperature history indictor formed as a label. This label indicator comprises a substrate for holding (1) microcapsules enclosing a hydrophobic organic compound which has a melting point, (2) a methine dye-precursor and (3) an oxidizing material.
Continuing to further improve the indicator, the inventors have found that the following problems are still left unsolved when the indicator is practically used.
Firstly, the conventional temperature history indicators are high in manufacturing cost and troublesome in treating them when they are to be used.
Secondly, the manner of storing and controlling matters is different from every matter and some of these matters may be left under a temperature higher than their permissible ones when they are being transported or moved, providing that the time period during which they are left under this temperature is extremely short. It is asked in this case that the color developing system is not worked, but none of the conventional temperature history indicators pays attention to this point and can distinguish the permissible rise of temperature, thereby causing their use to be limited.
Thirdly, when the indicator is stored under high humidity or at a place where dewing, freezing or the like is caused by temperature change or where adherence of oil, other chemicals or compounds is likely to be caused, water, oil or others adhered enter into the indicator to damage its temperature indication function. It was imagined at first that this could be solved by bonding a surface protection layer onto the whole surface of the microcapsule layer. However, it was found in a further study that the hydrophobic organic compound interacted with a surface protection material bonding agent to lower the melting point and that it was also absorbed by the bonding agent to lower the temperature indication function of the indicator, for example.